Two primary types of heart valve replacements or prostheses are known. One is a mechanical-type heart valve that uses a ball and cage arrangement or a pivoting mechanical closure supported by a base structure to provide unidirectional blood flow, such as shown in U.S. Pat. No. 6,143,025 to Stobie, et al. and U.S. Pat. No. 6,719,790 to Brendzel, et al., the disclosures of which is hereby expressly incorporated by reference. The other is a tissue-type or “bioprosthetic” valve having flexible leaflets supported by a base structure and projecting into the flow stream that function much like those of a natural human heart valve and imitate their natural flexing action to coapt against each other and ensure one-way blood flow. One example of a flexible leaflet valve is disclosed in U.S. Pat. No. 6,585,766 to Huynh, et al., the disclosure of which is hereby expressly incorporated by reference.
In tissue-type valves, a whole xenograft valve (e.g., porcine) or a plurality of xenograft leaflets (e.g., bovine pericardium) typically provide fluid occluding surfaces. Synthetic leaflets have been proposed, and thus the term “flexible leaflet valve” refers to both natural and artificial “tissue-type” valves. Two or more flexible leaflets are mounted within a peripheral support structure that usually includes posts or commissures extending in the outflow direction to mimic natural fibrous commissures in the native annulus. Components of the valve are usually assembled with one or more biocompatible fabric (e.g., Dacron) coverings, and a fabric-covered sewing ring is provided on the inflow end of the peripheral support structure.
In most bioprosthetic-type valves, metallic or polymeric structure provides base support for the flexible leaflets, which extend therefrom. One such support is an elastic “support frame,” sometimes called a “wireform” or “stent,” which has a plurality (typically three) of large radius cusps supporting the cusp region of the flexible leaflets (i.e., either a whole xenograft valve or three separate leaflets). The ends of each pair of adjacent cusps converge somewhat asymptotically to form upstanding commissures that terminate in tips, each extending in the opposite direction as the arcuate cusps and having a relatively smaller radius. The support frame typically describes a conical tube with the commissure tips at the small diameter end. This provides an undulating reference shape to which a fixed edge of each leaflet attaches (via components such as fabric and sutures) much like the natural fibrous skeleton in the aortic annulus. Other “support frame” constructions exhibit sheet-like tubular shapes but still define commissures and cusps on their outflow ends, such as shown in U.S. Pat. No. 5,984,973 to Gerard, et al., the disclosure of which is hereby expressly incorporated by reference.
The most common locations at which prosthetic heart valves are implanted are the aortic and mitral valves associated with the left ventricle, which generates the higher pressures of the two ventricles. The anatomy of the aortic and mitral valves are quite different, with the former being a tri-leaflet structure, and the latter having two dissimilar leaflets. The aortic annulus defines an undulating fibrous structure around substantially a circular ring to support the up-and-down shape of the cusps and commissures of the native leaflets. On the other hand, the mitral annulus has a somewhat fibrous linear anterior aspect across the septum of the heart from the aortic valve, but the posterior aspect, which defines the majority of the annulus, is relatively less fibrous and more muscular. Also, the posterior aspect is nearly oval-shaped such that the mitral annulus presents a malformed “D” shape. Furthermore, the mitral annulus does not exhibit the up-and-down undulating shape around its periphery as does the aortic annulus, and is generally planar, although the anterior aspect thereof is contoured in that it is generally lower at the fibrous trigones and rises up in-between to form something of a saddle shape. The present invention is directed to prosthetic heart valves particularly suited for implant at the mitral annulus.
The left ventricle LV and valves associated therewith are depicted in FIG. 1 as seen in vertical cross-section along an anterior-posterior plane. The mitral valve MV controls flow between the left atrium LA and the left ventricle LV, while the aortic valve AV functions between the left ventricle LV and ascending aorta AA. Both the mitral valve and aortic valve include leaflets that extend into the blood flow path and are supported around their peripheries by respective annuluses. For purpose of discussion, the mitral annulus of a normal, healthy heart lies generally in a mitral annulus plane MAP defined perpendicular to the average blood flow direction 20 (see FIG. 1) through the mitral valve MV. Although a typical mitral annulus may be three-dimensional, the mitral annulus plane MAP will be used as a reference plane that extends through the anterior and posterior aspects of the annulus. Papillary muscles are not shown but attach to the lower portion of the interior wall of the left ventricle LV and chordae tendineae extend between and link the papillary muscles and free edges of the anterior and posterior leaflets.
FIG. 2 illustrates the mitral valve from the left atrium as exposed during surgery, while FIG. 2A schematically identifies the commonly identified features in plan view. The average human mitral annular cross-sectional area is 5-11 cm2. The anterior aspect of the mitral annulus forms a part of the “cardiac skeleton” and includes anteriolateral ALT and posteriomedial PMT fibrous trigones. The mitral valve MV is a bicuspid valve having a posterior leaflet (separated into three cusps P1, P2, P3) that cooperates or coapts with an anterior leaflet A. The anteriolateral ALT and posteriomedial PMT trigones are indicated at the junction points of the anterior leaflet A and first and third cusps of the posterior leaflet P1, P3, respectively. These junction points are also known as commissures between the leaflets.
As mentioned above, and with reference to FIG. 2A, the mitral annulus has a mal formed “D” shape with the straight portion, or anterior aspect, extending between the anteriolateral ALT and posteriomedial PMT trigones. The longest dimension of the mitral annulus defines a major axis 22 while the shortest dimension defines a minor axis 24. The minor axis 24 generally bisects the anterior leaflet A. Although the mitral annulus is not circular, a center 26 thereof can be defined at the intersection of the major and minor axes 22, 24. Radial lines can be drawn outward from this imaginary center 26 through the anteriolateral ALT and posteriomedial PMT trigones to indicate the angular separation φ therebetween. This angular separation φ varies from patient to patient, but is generally about one-third of the circumference around the mitral annulus, or 120°.
Numerous prosthetic heart valves have been proposed in the art, typically without too much consideration of the unique attributes of the different valve annuluses. In particular, they have been almost no attempts to modify prosthetic valves to better conform to the mitral annulus.